This invention relates to the replacement of tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer wire insulation by tetrafluoroethylene/hexafluoropropylene copolymer wire insulation.
EP 0 423 995 discloses the fluorine treatment of tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (PFA) and tetrafluoroethylene/hexafluoropropylene copolymer (FEP) to reduce the dissipation factor for these copolymers, enabling cable using these copolymers as insulation for the conductor in the cable to exhibit reduced attenuation of the transmitted signal. As shown in Table 1, dissipation factor for the FEP at 1 GHz is reduced from 0.00112 to 0.00057 by fluorination, and for PFA at 500 MHz, the dissipation factor is reduced from 0.00083 to about 0.000366 by fluorination. According to U.S. Pat. No. 4,743,658, which is referred to in '995 for the fluorination process, the fluorinated PFA contains less than 6 unstable end groups per 106 carbon atoms. Similar results are obtained when FEP is fluorinated by the process of the '658 patent. For both copolymers, the fluorination converts the polymer unstable end groups, typically —COF, —CONH2, —COOH, and/or —CH2OH, resulting from the copolymerization process to the stable end group —CF3. PFA exhibits a lower dissipation factor than FEP both before and after fluorination, and the dissipation factor reduction for the PFA arising from fluorination is greater than for the FEP. As a result of this difference in dissipation factor as well as the difference in melting points (PFA melts at about 305-310° C. and FEP melts at about 250-255° C.), these copolymers have found different utilities as wire insulation.
Especially for the FEP, steps have been taken to greatly increase line speed in the extrusion coating operation. U.S. Pat. No. 5,677,404 discloses FEP compositions that improve line speed. U.S. Pat. Nos. 6,541,588 and 6,623,680 disclose the processing condition at the time of fluorination to improve line speed. U.S. Pat. No. 6,743,508 discloses an increase in adhesion between the wire and the FEP insulation to improve line speed. The trend in these patents is to achieve line speeds of 500 m/min and higher, using FEP having a melt flow rate (MFR) of at least about 24 g/10 min to produce a solid insulation over wire.
Recently, there has developed a desire to replace foamed PFA wire insulations by foamed FEP insulation in coaxial cable, because FEP is a less expensive copolymer than PFA. The production of foamed insulation coaxial cable is different from the high line speed production of solid wire insulation, arising from the cable construction and the foaming process, resulting in low line speeds, i.e. less than 90 ft/min (27.4 m/min) in commercial operation for the most common dimension coaxial cable wherein the central conductor is 0.032 in (0.8 mm) in diameter and the foamed insulation is 0.135 in (3.4 mm) in diameter, void content being about 55%. Low MFR FEP, i.e. MFR of about 7 g/10 min, has been used to produce the foam and to have the coaxial cable pass the NFPA 262 test, with respect to resistance to burning and smoking. This FEP is FEP A of the Examples. The low MFR FEP resists dripping and therefore is sufficiently non-smoking that the coaxial cable passes the NFPA 262 test, in contrast to high MFR, which can only be foamed at very low line speed to enable the foam to set up upon exit from the extruder and which drips and smokes when the coaxial cable containing this FEP is subjected to the NFPA 262 test, causing such cable to fail this test.
While coaxial cable containing foamed FEP insulation made from low MFR FEP can be made, which passes the NFPA 262 test, such cable suffers from the disadvantage that it has an unacceptably high return loss of at least about −20 dB as compared to the return loss of about −24 dB that is achievable with coaxial cable having foamed PFA as the insulation. These return losses are the general results obtained in commercial practice. Occasionally better return loss can be achieved with this FEP foamed insulation, but this is unsustainable on a continuing or reproducible basis. The higher the return loss, the smaller is the negative numerical value, e.g. −19 dB is a higher return loss than −20 dB. The effect of the higher return loss for the foamed FEP insulation is that this insulation cannot replace PFA as the foamed insulation in coaxial cable in applications requiring low return loss in the range 800 MHz to 3 GHz. The return losses mentioned above and hereinafter are measured across this range and averaged as further described in the Examples, this being the preferred frequency range for measurement to achieve widespread use for the coaxial cable. The 3 GHz frequency is merely a convenient stopping point for the frequency sweep, starting from 800 MHz; the sweep can continue to even higher frequencies than 3 GHz, without appreciably affecting the average return loss measurement result. FEP has the additional disadvantage as compared to PFA of higher dissipation factor than PFA. Both higher dissipation factor and high return loss contribute to increased loss (attenuation) of the signal transmitted by the coaxial cable.
Thus, FEP foamed insulation has not been able to replace PFA foamed insulation in coaxial cable applications requiring the return loss to be at least as good as when the PFA foamed insulation is used at 1 GHz.